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Any way to determine the design voltage of a random miniature DC motor?

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  • #16
    For the low cost permanent magnet DC "toy" motors from Mabuchi and most others I've never seen a rotor with anything other than three lobes. So that means six commutator contacts. Mind you there may be some other higher tech options in his collection.

    Chilliwack BC, Canada


    • #17
      Originally posted by BCRider View Post

      I'm also not sure that running such motors off a hand drill to act as a generator will tell the whole story. The higher torque motors will have fewer turns of heavier wire. The lower torque motors will generally have more turns of finer wire. The greater the number of turns the more voltage they will generate when driven at a given RPM. But both might be rated to run on the same voltage. Comments from those that suggest the generator approach are sought on this. But what I just wrote is how I understand this to work.

      At low voltages things are not as they are at 90V or 180V. But............. you are in the right direction

      Wire size indicates current capability. There are many reasons that a motor might have large wire and fewer turns. Low voltage and higher power, very high speed with decent low speed torque (often with a controller, as for a spindle motor), higher power just in general, etc, etc. The motors the OP seems to be describing are ones likely with no special design, and in that case wire size is basically voltage or power or both.

      The generated voltage depends on magnetic flux and turns and rpm. A big magnet may give higher voltage with fewer turns at a given rpm. A smaller magnet and more turns may give the same.

      The voltage generated opposes the applied voltage from the battery etc. The difference between the applied and generated (back EMF) voltages is what pushes current through the motor resistance.

      That's how the motor increases torque with load.... it slows a little, which reduces the back EMF, and allows more current to flow, creating more torque. The motor automatically finds a balance if it can. If not, it stalls. A PM or shunt DC motor has a maximum speed, which is where the back EMF just allows enough current to overcome friction and windage.

      There are a lot of ways to design a motor. Cheap motors are designed to be cheap, so they balance magnet, mass of winding wire, motor iron weight, etc to get an economical motor that probably is not very efficient. They tend to be more turns and less magnet, and so less power, but that is not a law.

      Bottom line is that the appearance really does not tell you much. Some small cheap motors tend to run on 3 to 6 V and battery currents. Larger ones may run on 6 to 24V, but that range is not so helpful, probably.

      Size does not tell you much, wire size may "suggest" a voltage or power range.

      Running the motor is a good test, especially if you check current. Doing a generator test is a good test. Those may let you guesstimate what you have.

      Most small motors are not very critical, and nearly all of them run without much heat rise, since they have no special insulation, maybe not even ordinary 60C insulation. A hot small motor, too hot to touch, is probably being abused. Warm is fine.


      Keep eye on ball.
      Hashim Khan

      Everything not impossible is compulsory


      • #18
        You mentioned Maxon- this could be a three phase brushless motor, and these may run up to 60,000 rpm. If it's a brush motor, then that is unlikely. Here's what I would like you to do- lay all these motors out on a table in rows so we could say for instance 'second row, third from left'- then we're likely to be able to say what voltage they were designed to run from. Motors that come from VCRs for instance usually are 12 volt, as are virtually all motors from anything automotive. If they are brush motors from printers, very often they are 24 volts. If they are brushed motors with wound fields and armatures, usually 110 vac. Many of them will be identifiable by case size, shape, and cutouts. Motors from cordless drills will seldom spin at less than 10,000 rpm at their rated voltage, and many will spin at 15 or 16000 rpm unloaded at rated voltage. One very rough rule of thumb for pm motors is that they will begin to spin at a voltage that is about 10% of their rated voltage. For instance if a motor just begins to turn at 1.2 volts, it's likely to be a 12 volt motor. If it takes 8 or 10 volts to just begin to turn, it's probably a 90 volt motor. If it's revving pretty good at 1 volt or less, it's probably a 3 volt or maybe a 5 volt motor. I've seen so many small motors I should be able to identify many if not most of them.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


        • #19
          Another thing which you could do, although this is more complicated- many of these motors will have an idle current which rises slightly as rpm increases. This current will begin to rise faster as you begin to exceed the design rpm. Sensing this 'knee' point can give an indication of the design voltage. This will be difficult to do unless you have a variable power supply of good design- and preferably one with analog meters. And you will have to take into account the condition of the motor. I almost invariably put some TriFlow into the bearings to make sure I'm not being tricked by dry bearings. Also, if there's much of a groove at all on the commutator, the motor is probably not worth trying to use for something, even if it seems to run fine. Same if the brushes are worn to any real extent- and especially if they are worn to different lengths. Unless you're hopelessly hooked on rebuilding it, just toss those out.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


          • #20
            Increase the voltage until the temperature of the motor starts increasing. A good place to operate a motor is where the temperature is stable. If the temperature starts increasing, your voltage is too high.


            • #21
              Originally posted by SLK001 View Post
              Increase the voltage until the temperature of the motor starts increasing. A good place to operate a motor is where the temperature is stable. If the temperature starts increasing, your voltage is too high.
              The problem with this method is that the cheaper small toy style motors will run cool at WELL past their intended operating voltage. Running them with no load does not pull much current. So you can get a 6 volt motor to run cool at 12volts. But any load on it and the current will rise quickly and cause it to overload and let out the magic smoke. But because it'll be running at well over the rated RPM it won't have a happy purr to it.

              Darryl's post about giving us a picture is a good one. There's small cheap can motors intended for toys and light appliance use and there's some fancy small motors that can end up being intended for more special purposes. Some of the fancier motors folks might recognize and have data for you to try.

              I also like his option for testing to find the minimum voltage that starts them running. That's a pretty good indicator for the cheaper "toy style" light duty brushed motors.
              Chilliwack BC, Canada